Transmissions are used to vary the ratio of output torque to output angular velocity in systems which transmit power by exerting the product of torque and angular velocity. Typically, a transmission is employed in the drive train of a machine and has an input shaft which is selectively coupled to a drive source and an output shaft which is coupled to a driven source whereby the output of the drive source is utilized to do work. The power output of a typical drive source, e.g., an internal combustion engine, generally increases with the speed of operation of the drive source as reflected by an increasing torque and rate of rotation of the input shaft. The rate of rotation of the output shaft generally determines the speed of operation or output angular velocity of the machine. Thus, the ratio of torque to angular velocity is changed by varying the transmission ratio of the transmission, i.e., the ratio of the output rate of rotation to the input rate of rotation. Transmissions are therefore utilized to approximately match, within the constraints of the transmission and drive source, the delivered energy to existing operating conditions, including the load on the output shaft and machine operating speed.
The case of a common automotive transmission is illustrative. The transmission input shaft is coupled to the automobile engine and the output shaft is coupled to the drive wheels of the automobile. Automobile transmissions are often capable of providing several discrete transmission ratios which are selectively employed to suit driving conditions. For example, in some cases, such as accelerating from a stop or climbing a hill, a relatively large torque-to-angular-velocity ratio is required and a low transmission ratio can be utilized. Conversely, as the automobile speed increases, a smaller torque-to-angular-velocity ratio may be necessary and a higher transmission ratio can be selected. In this regard, many modern automobiles include an overdrive transmission ratio, i.e., a ratio greater than unity or thereabouts such that the output shaft rotates faster than the input shaft. Such a transmission ratio can be employed, for example, in constant speed highway driving conditions wherein the overdrive ratio allows delivery of sufficient torque to maintain constant automobile speed while allowing the engine speed to be reduced relative to one-to-one drive, thereby saving fuel and reducing engine wear.
In some conventional transmissions, the transmission ratio is varied by disengaging a first set of torque wheels and engaging a second set of torque wheels. As used herein, the term "torque wheel" includes gears, sprocket wheels, pulleys and other rotatable members utilized to transmit power by exerting torque and angular velocity. Appropriate linkage, e.g., gear teeth, belts, chains, or fluid couplings are provided to link torque wheels so that torque is transmitted therebetween when the torque wheels are engaged. As between torque wheels, speed reduction generally means torque increase. The torque transmitted from a driving torque wheel to a driven torque wheel is therefore dependent upon the relative rates of rotation of the wheels when engaged which, in turn, depends on the torque wheel ratio of the wheels, e.g., the relative number of gear teeth, sprocket teeth or pitch diameters, of the wheels. Thus, changing the transmission ratio in conventional transmissions commonly involves disengaging a first set of torque wheels and engaging a second set of torque wheels having an overall torque wheel ratio different from that of the first set of wheels.
Relatedly, transmissions are generally used in conjunction with clutch mechanisms in the drive trains of machinery. Clutch mechanisms serve to uncouple the drive source and driven source, i.e., to allow rotational slippage therebetween. A clutch mechanism can be used to uncouple the drive source and driven source so as to allow idling and starting of the drive source without an output load thereon. It will be appreciated that such a load will be incurred when a transmission is engaged in a non-zero transmission ratio fashion. Similarly, clutch mechanisms are used to uncouple the drive source and driven source while the transmission ratio is changed by disengaging a first set of torque wheels and engaging a second set of torque wheels having an overall torque wheel ratio different from that of the first set of torque wheels.
Although various types of clutch mechanisms have been developed over the years, such mechanisms commonly entail frictional losses. By way of example, one known type of clutch mechanism utilizes a pair of opposing friction plates, one of which is associated with the drive source and the other associated with the driven source. To uncouple the drive source and driven source, the plates are operatively separated so that the plates can rotate at different rates. To couple the drive source and driven source, the plates are urged together in frictional contact so that the plates rotate in unison. Both static and dynamic frictions are involved. Such frictional coupling then results in loss of useful power and increased machinery wear.
There are a number of drawbacks or less than optimized aspects associated with transmissions such as identified above. Initially, such transmissions can achieve only a limited number of discrete transmission ratios or a narrow range of transmission ratios. Therefore, the transmission ratio of such devices can only be roughly varied to suit changing operation conditions resulting in un-optimized power utilization and/or reduced drive source efficiency. Additionally, with such transmissions it is generally necessary to use a clutch mechanism to uncouple the drive source and driven source because a transmission ratio of zero cannot be achieved when the torque wheels are engaged, e.g., when the gears are meshed. Accordingly, it would be advantageous to provide a transmission wherein the transmission ratio is infinitely or steplessly varied across a range of transmission ratios including zero, and wherein torque wheels remain continuously engaged, thereby enhancing machinery performance and efficiency, reducing machinery friction and wear, and substantially eliminating the need for a clutch mechanism.
Heretofore, transmissions have been provided wherein the effective torque wheel ratio between engaged torque wheels was variable across a limited range. For example, belt and pulley type transmissions have been constructed wherein the torque wheel ratio between two pulleys could be varied by causing the belt to ride higher or lower within the flanges of at least one of the pulleys such that the effective radius of the pulley was changed. Thus, the transmission ratio of such a transmission could be reduced by increasing the effective radius of a pulley coupled with the transmission output or by decreasing the effective radius of a pulley coupled with the transmission input. However, it is readily apparent that such devices cannot provide a transmission ratio of zero without disengaging torque wheels as this would require an infinitely large output pulley or an input pulley having a radius of zero. Such devices may also be subject to catastrophic failure due to frictional contact between the belt and pulley flanges where again dynamic friction loss is unavoidable. Thus, it would be advantageous to provide a transmission wherein the transmission ratio can be infinitely varied without varying the torque wheel ratio of engaged torque wheels.